All good, either way, my friend. I'm not one for taking the word of authorities or so-called authorities as gospel, regardless of their creds. Good information gathering is indeed about an informed consensus. And yet, if we take a consensus on a topic like this, you'll find that most folks do not reconstitute their demineralized water. Which would be fine if they could justify it, but like so much of hobby habits...its founded on misinformaton or an outright lack, thereof.
At the end of the day, what matters is adequate and stable mineral levels in the aquarium. And the fundamentals of what we are talking about here is maintaining adequate if not strong pH and alkalinity for better rates of calcification, larger bioloads, or simply the burden (steadily increasing dissolved organic) of a maturing tank in time).
These realities, made worse still by not uncommon scenarios such as a lack of water changes (reflecting the impact of unabated production of organic/nitric acids fro bioactivity), use of calcium reactors (CO2/carbonic acids also chewing up alkalinity)...
Erhhhm, I could go on. But you get the picture. The nature of aquarium keeping (all...fresh salt and brackish of course) is an acidic slide. We have to maintain pH and alkalinity (so much more important for pH stability) for the system we choose to set up.
Compound this by the reality that a lot of buffers are made, well...cheaply. Relying principally on sodium bicarbonate which was fine as a staple for marine aquarium keeping in the 1970's (example) when the few organisms hardy enough to survive archaic import and holding systems would in fact live in an aquarium sadly maintained with baking soda as a buffer (read: pH levels chronically under 8.0 and ultimately never able to go higher than 8.3). Its no coincidence that buffer manufacturers back then conveniently misreported (ahem) and promoted/advertised (cough) that a healthy marine aquarium should be kept between 8.0 and 8.3. To offer a better buffer than relabeled baking soda would have cost more, made less (profit) and sold less to tiny market of consumers not ready to spend big bucks like we do today. Ironic historical aside: the pushers of cheap buffer would ultimately place the responsibility for buffering on dolomite substrates or calcite...ironic because they do not dissolve readily until the pH drops to around 7.6. LOL (at which point you have a bigger problem than quantifying which was the better buffer, your cheap alkalinity booster or the crappy substrate you just bought.) [Note: aragonite will dissolve at a pH over 8.0...well over actually. Read: a better buffer support if you choose to rely on substrate for part of the duty]
So, we (hobbyists) shop and debate and argue online about sea salts and which has the higher pH and which has better Ca or Alk on mixing...without talking about the source water we are using to mix it up? Erhhhhm...?!?
And salt some brands of which, albeit tried and true - all respect indeed - have not fundamentally changed their recipe in a gazillion years. Meaning...they formulated these salts at a time when almost nobody used purified water.
At the end of the day, it really doesn't matter how we buffer the water and insure better pH and alkalinity, so long as it gets done safely and hopefully consistently over time. And yes, there are sea salts that are formulated for use with RO or otherwise demineralized water.
But a majority of aquarists struggle with pH problems; very few consistently have a ph that doesn't drop below 8.3 at night let alone above it by day! And if you add raw, demineralized (RO/DI) evap water to your souring reef gallon by gallon day after day...how to you reckon that amelioration of water with 300+ ppm mixing with water with near zero ppm of hardness? For example...10 gallon (actual volume) of sump water at 10dKH gets 1.5 gall of evap top offf water each day with a pH under 7 and a dKH near nil, what do we expect the readings to be after the mix? Lessoned, indeed by the total volume of the system (maybe a 90 gall display this example). But day after day raw, sour, "pure" RO?DI water giong in and adding to the overall pressure on the system to keep pH and Alk up.
The simple fact is that demineralized water (RO/DI) is extremely unstable at any pH it comes to you (even sometimes even it is over 7). By aerating it (driving off CO2/carbonic acid) you will raise the temporary pH. And by buffering it slightly, you ease the burden and hedge your bets on the inevitable processes in aquaria ongoing after your water exchange that will bring down pH and alkalinity. There is no disadvantage to remineralizing your ro/Di water before it is used for evap top off or salting.
That is, I think...my short answer
FWIW, I do still use the old fashioned "tri-buffer recipe" (bicarb, carb and a borate) for dirty or bulk work...but I prefer to use well engineered products from thoughtful companies like Brightwell Aquatics and Seachem for this purpose.
